FIG. 1 depicts a prior art device having a single-element hydrophone 7. This device has a small finger-like electrode 4 (“finger electrode 4”) and a large area electrode 3 (“LA electrode 3”). Each of the electrodes is attached to a different side of an electret film 2 (“E film”). The molecules of the E film 2 are aligned so as to create a permanent electrostatic field. The two electrodes 3, 4 with the E film 2 between form a capacitor. When the distance between the electrodes 3, 4 changes, the voltage differential between the electrodes 3, 4 changes. The change in voltage may be detected in order to signal when pressure is being applied to the one or both of the electrodes 3, 4. In this manner, a longitudinal wave, such as an acoustic wave, may be detected.
In the prior art, a tip-portion 4A of the finger electrode 4 is positioned relative to the LA electrode 3 such that the distance from the tip-portion 4A of the finger electrode 4 to the LA electrode 3 is the same as the thickness of the E film 2. The E film 2 may be substantially flat and thereby defines a plane, and when viewed in a direction perpendicular to the plane, the electrodes 3, 4 may be thought of as having an overlapping area, which is shown in FIG. 1 as cross-hatched areas on each electrode 3, 4.
The overlapping area, and the E film 2 residing in that area, is commonly described as a hydrophone element 7. A first electrical connection (not shown) is made with the finger electrode 4, and a second electrical connection (not shown) is made with the LA electrode 3 in order to permit the hydrophone element 7 to detect acoustic energy impinging on the finger electrode 4 or impinging on the LA electrode 3.
Such prior art devices are often fabricated using a piece of E film 2 that is the size of a small shirt button. In such devices, the hydrophone element 7 may be a few thousandths of an inch wide. The electrodes 3, 4 are commonly formed using standard masking and sputtering techniques. In order to simplify manufacturing, the LA electrode 3 typically covers half of one side of the film 2.
The prior art devices, because of limited use and design, do not lend themselves well to mass production and are normally hand-made by skilled technicians in a very labor intensive manner.